Developmental toxicants including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and chlorophenols (e.g., pentachlorophenol - PCP) are persistent organic contaminants found at many hazardous waste sites throughout the United States. These contaminants are of great concern because they present high toxicological risk in terms of their potential for bioaccumulation in the food chain as well as adverse health effects. Hydrophobic organic contaminants (HOCs) such as PAHs, PCBs and PCP are particularly challenging to bioremediate because their chemical characteristics diminish their bioavailability to microorganisms thereby limiting their potential for efficient biodegradation. As a result of this constraint, HOC remediation strategies tend to focus on physical-chemical approaches consisting of either complete excavation of contaminated media or contaminant in situ immobilization via amendment mediated sequestration which can have significant negative long term impacts on local ecosystems. In this project, we propose an alternative treatment approach which consists of stimulating cooperative bacterial-fungal biofilms for HOC biodegradation. Using this approach, indigenous fungi will first be stimulated to degrade toxicants using nonspecific extracellular enzymes and generate by-products more bioavailable to bacteria for subsequent biodegradation. While using remediation treatment approaches based on the production of fungal extracellular enzymes is not entirely new, the novelty of this project resides in the fact that our focus will be directed towards the promotion of indigenous Ascomycetes associated with sediment microbial biofilms, a phylum which has not received much attention for bioremediation. The overall objective of this project is to maximize HOC biodegradation in sediment settings by stimulating the growth of synergistic fungal-bacterial biofilms using engineered composite organic amendments. The general hypothesis for Project 5 is that the physico-chemical environment can be altered using composite organic amendments to stimulate the formation of a cooperative bacterial-fungal biofilms in which indigenous fungi produce extracellular enzymes breaking down hydrophobic contaminants into by-products which are more readily transported into bacterial cells and broken down by indigenous bacteria. The specific aims for this project are to: 1) Perform microbial and geochemical characterizations of contaminated sediments for the construction of a microbial association network model; 2) Identify organic amendments which support the growth of cooperative fungal-bacterial biofilm and maximize HOC degradation in microcosms; 3) Engineer and test composite amendments for delivery in sediment treatment scenarios and; 4) Implement the composite amendment strategy in large-scale mesocosms as well as validate its efficiency for HOC degradation by stimulated mixed fungal-bacterial biofilms. Ultimately, this project will create a framework to better understand cooperative fungal-bacterial biofilms in the context of sediment bioremediation and yield field translatable approaches for the bioremediation of HOCs.